Coastal Wetlands

Maritime Studies, 2019

The atmosphere and the ocean exchange gases resulting in an overall absorption of CO 2 in the ocean Burning fossil fuels, industry and changes in land uses (deforestation, landfill , fires and/or agriculture), and respiration by humans and animals releases extra CO 2 and CH 4 to the atmosphere Plants within terrestrial forests, tidal marsh, mangrove and seagrass ecosystems sequester CO 2 through photosynthesis, which accumulates in their biomass and soils CO 2 , CH 4 CO 2 , CH 4 CO 2 CO 2 CO 2 CO 2 CO 2 CO 2 CO 2 CO 2 Photosynthesis by plankton in the ocean sequester Export of plant and macroalgal biomass in the deep ocean sequesters CO 2 FIGURE 28.1 Conceptual diagram of carbon sequestration by blue carbon ecosystems and some of the activities that influence CO 2 exchange among the atmosphere, soil, and ocean in coastal areas and the open ocean. The major global C pools include the atmosphere, oceans, fossil fuels, vegetation, soils, and detritus. Landfill, smokestacks, cattle farming, and other human activities result in additional methane (CH 4) emissions. 28. CONSERVATION OF BLUE CARBON ECOSYSTEMS FOR CLIMATE CHANGE 966 VII. COASTAL WETLAND SUSTAINABILITY seagrass ecosystems occupying less than 0.2% of the seabed area, they contribute nearly 50% of the CO 2 sequestration in marine sediments, and their C sequestration rates exceed those in the soils of many terrestrial ecosystems by 30-to 50-fold (Chmura et al., 2003; Duarte et al. 2005, 2013; Mcleod et al., 2011). Most macroalgal communities grow on rocky substrate and do not form significant in situ sedimentary C deposits, but the initial estimates of the amount of macroalgae C sequestered in sediments and deep-sea waters suggest that it is comparable to the C sequestered by all other BC ecosystems combined (Krause-Jensen and Duarte, 2016). Furthermore, the C captured by BC ecosystems is stored in marine soils for millennia, rather than the decades or centuries typical of terrestrial forests. This is due in part to the high rates of vertical accretion in tidal marsh, mangrove, and seagrass ecosystems, ranging from 0.4 to 21 mm year À1 (Mateo et al., 1997; McKee et al., 2007; Duarte et al., 2013). This process of raising the seafloor is driven partly through the trapping and settling of particles from the water column and partly through organic matter production. This acts to bury the C in anoxic conditions, thereby slowing down its remineralization by microbes (Krauss et al., 2014; Mateo et al., 2006; Pedersen et al., 2011). Globally, tidal marsh, mangrove, and seagrass ecosystems sequester annually a similar amount of C to terrestrial forests, despite their extent being less than 3% of that of forests (Duarte et al., 2013). Unlike terrestrial forests, mangroves and tidal marshes rarely burn in wildfires, although they are exposed to other disturbances (e.g., tropical storms). BC ecosystems provide important and valuable ecosystem services critical for climate change mitigation and adaptation, including coastal protection from storms and shoreline erosion, regulation of water quality, provision of habitat for commercially important fisheries and enhancing biodiversity, and being globally significant C sinks (

Wetlands, 2017

Across the globe, coastal wetland vegetation distributions are changing in response to climate change. In the southeastern United States, increased winter temperatures have resulted in poleward range expansion of mangroves into pure salt marsh habitat. Climate change-induced expansion of mangroves into salt marsh will significantly alter carbon (C) storage capacity of these wetlands, which currently store the highest average C per land area among unmanaged terrestrial ecosystems. Total ecosystem C stocks were measured along a 342 km latitudinal gradient of mangrovetomarsh dominance in Florida. Carbon stocks were quantified through measurements of above-and belowground biomass and soil C. Interior mangrove C stocks were greater than both salt marsh and ecotonal C stocks and soil C comprised the majority of each ecosystem C component (51-98%). The wetlands investigated in this study cover 38,532 ha, and store an average of 215 Mg of C ha −1. Currently, mangroves cover 31% of the land area studied, storing 44% of the total C, whereas salt marshes occupy 68% of the wetland area and only store 55% of the C. Total conversion of salt marsh to mangrove may increase C storage by 26%, predominately due to increases in aboveground biomass.

Proceedings of the National Academy of Sciences of the United States of America, 2016

Given their relatively small area, mangroves and their organic sediments are of disproportionate importance to global carbon sequestration and carbon storage. Peat deposition and preservation allows some mangroves to accrete vertically and keep pace with sea-level rise by growing on their own root remains. In this study we show that mangroves in desert inlets in the coasts of the Baja California have been accumulating root peat for nearly 2,000 y and harbor a belowground carbon content of 900-34,00 Mg C/ha, with an average value of 1,130 (± 128) Mg C/ha, and a belowground carbon accumulation similar to that found under some of the tallest tropical mangroves in the Mexican Pacific coast. The depth-age curve for the mangrove sediments of Baja California indicates that sea level in the peninsula has been rising at a mean rate of 0.70 mm/y (± 0.07) during the last 17 centuries, a value similar to the rates of sea-level rise estimated for the Caribbean during a comparable period. By accr...

Nature, 2019

Coastal wetlands (mangrove, tidal marsh and seagrass) sustain the highest rates of carbon sequestration per unit area of all natural systems 1,2 , primarily because of their comparatively high productivity and preservation of organic carbon within sedimentary substrates 3. Climate change and associated relative sea-level rise (RSLR) have been proposed to increase the rate of organic-carbon burial in coastal wetlands in the first half of the twenty-first century 4 , but these carbon-climate feedback effects have been modelled to diminish over time as wetlands are increasingly submerged and carbon stores become compromised by erosion 4,5. Here we show that tidal marshes on coastlines that experienced rapid RSLR over the past few millennia (in the late Holocene, from about 4,200 years ago to the present) have on average 1.7 to 3.7 times higher soil carbon concentrations within 20 centimetres of the surface than those subject to a long period of sea-level stability. This disparity increases with depth, with soil carbon concentrations reduced by a factor of 4.9 to 9.1 at depths of 50 to 100 centimetres. We analyse the response of a wetland exposed to recent rapid RSLR following subsidence associated with pillar collapse in an underlying mine and demonstrate that the gain in carbon accumulation and elevation is proportional to the accommodation space (that is, the space available for mineral and organic material accumulation) created by RSLR. Our results suggest that coastal wetlands characteristic of tectonically stable coastlines have lower carbon storage owing to a lack of accommodation space and that carbon sequestration increases according to the vertical and lateral accommodation space 6 created by RSLR. Such wetlands will provide long-term mitigating feedback effects that are relevant to global climate-carbon modelling. Broad biogeographic drivers, such as vegetation, climate, topography or water chemistry, are often emphasized as important global-scale controls on organic matter accumulation, decomposition and carbon stocks within tidal wetlands 7. However, relative sea-level trends over the Holocene varied across the globe, principally on the basis of distance from maximal ice-sheet extent during the last glacial period, and have a profound influence on the contemporary character of coastal wetlands 8,9. In Europe and North America, where studies of coastal wetland sea-level rise (SLR) impacts are concentrated, sea levels have been rising over the past few millennia at a decelerating rate up to the present (Fig. 1a, b). Tidal marshes in these locations, particularly when sediment supply is low-moderate, are often characterized by deep sediments that are highly organic 4,10,11 , in contrast to coastal wetlands in locations where the sea level has been stable for the past few millenia 12 , in spite of similarities in floristics 13. We review published data, contribute new observations on soil carbon concentrations (%C) in tidal-marsh sediments and compare %C values over the active root zone (0-20 cm) and sub-surface depths (20-50 cm, 50-100 cm and >1 m) for 345 locations that vary in rates of SLR over the late Holocene (Supplementary Information). We find that variation in RSLR over past millennia is a primary control on carbon storage. Overall %C varies consistently between RSLR zones (areas spatially

Environmental Research Letters, 2018

Certain coastal ecosystems such as mangrove, saltmarsh and seagrass habitats have been identified as significant natural carbon sinks, through the sequestration and storage of carbon in their biomass and sediments, collectively known as 'blue carbon' ecosystems. These ecosystems can often thrive in extreme environments where terrestrial systems otherwise survive at the limit of their existence, such as in arid and desert regions of the globe. To further our understanding of the capability of blue carbon ecosystems to sequester and store carbon in such extreme climates, we measured carbon sediment stocks in 25 sites along the Western Arabian Gulf coast. While seagrass meadows and saltmarsh habitats were widely distributed along the coast, mangrove stands were much reduced as a result of anthropogenic pressures, with 90% of stands having been lost over the last century. Carbon stocks in 1 m deep surface sediments were similar across all three blue carbon habitats, with comparable stocks for saltmarsh (81 ± 22 Mg C org ha −1), seagrass (76 ± 20 Mg C org ha −1) and mangroves (76 ± 23 Mg C org ha −1). We recorded a 38% decrease in carbon stocks between mature established mangrove stands (91 Mg C org ha −1) and recently planted mangroves (56 Mg C org ha −1). Mangroves also had the lowest carbon stock per total area owing to their very limited spatial coverage along the coast. The largest stock per total area belonged to seagrass beds as a result of their large spatial coverage within the Gulf. We employed 210 Pb dating to determine the sediment accretion rates in each ecosystem and found mangrove habitats to be the most efficient carbon sequesters over the past century, with the highest carbon burial rate of the three ecosystems (19 g C org m −2 yr −1), followed by seagrass (9 g C org m −2 yr −1) and saltmarshes (8 g C org m −2 yr −1). In this work, we describe a comprehensive comparison of sediment stocks in different blue carbon ecosystems within a single marine environment and across a large geographical area, and discuss our results in a global context for other blue carbon ecosystems in the dry tropics.

Current Opinion in Environmental Sustainability, 2012

Coastal vegetated wetlands have recently been identified as very important global C sinks but vulnerable to degradation by direct human alteration of their habitats. While their expanse is small globally, areal rates of C burial, or sequestration, are among the highest of Earth's ecosystems. There is considerable uncertainty in the magnitude of total global sequestration in these systems for two reasons: poor estimates of their global areas and high variability and uncertainty in areal rates of burial between systems. The magnitude of C burial in vegetated coastal systems has been decreasing rapidly over the past century due primarily to human disturbances such as dredging, filling, eutrophication, and timber harvest. These systems continue to be lost globally at rates ranging from 1% to 7% annually. We find that climate change including global warming, human engineering of river systems, continued agricultural expansion, and sea level rise will also negatively impact C burial of coastal vegetated wetlands. A decrease in global C burial in these systems will ultimately exacerbate CO 2 emissions, and further contribute to climate change in the future.

National Science Review, 2020

Coastal tidal wetlands produce and accumulate significant amounts of organic carbon (C) that help to mitigate climate change. However, previous data limitations have prevented a robust evaluation of the global rates and mechanisms driving C accumulation. Here, we go beyond recent soil C stock estimates to reveal global tidal wetland C accumulation and predict changes under relative sea level rise, temperature and precipitation. We use data from literature study sites and our new observations spanning wide latitudinal gradients and 20 countries. Globally, tidal wetlands accumulate 53.65 (95%CI: 48.52–59.01) Tg C yr−1, which is ∼30% of the organic C buried on the ocean floor. Modeling based on current climatic drivers and under projected emissions scenarios revealed a net increase in the global C accumulation by 2100. This rapid increase is driven by sea level rise in tidal marshes, and higher temperature and precipitation in mangroves. Countries with large areas of coastal wetlands, ...

Science of The Total Environment

Coastal vegetated habitats can be important sinks of organic carbon (C org) and mitigate global warming by sequestering significant quantities of atmospheric CO 2 and storing sedimentary C org for long periods, although their C org burial and storage capacity may be affected by ongoing sea level rise and human intervention. Geochemical data from published 210 Pb-dated sediment cores, collected from low-energy microtidal coastal wetlands in El Salvador (Jiquilisco Bay) and in Mexico (Salada Lagoon; Estero de Urias Lagoon; Sian Ka'an Biosphere Reserve) were revisited to assess temporal changes (within the last 100 years) of C org concentrations, storage and burial rates in tropical salt marshes under the influence of sea level rise and contrasting anthropization degree. Grain size distribution was used to identify hydrodynamic changes, and δ 13 C to distinguish terrigenous sediments from those accumulated under the influence of marine transgression. Although the accretion rate ranges in all sediment records were comparable, C org concentrations (0.2-30%), stocks (30-465 Mg ha −1 , by extrapolation to 1 m depth), and burial rates (3-378 g m −2 year −1

Limnology and Oceanography, 1999

Effects of nutrient loading on the carbon balance of coastal wetland sediments Abstract-Results of a 12-yr study in an oligotrophic South Carolina salt marsh demonstrate that soil respiration increased by 795 g C m Ϫ2 yr Ϫ1 and that carbon inventories decreased in sediments fertilized with nitrogen and phosphorus. Fertilized plots became net sources of carbon to the atmosphere, and sediment respiration continues in these plots at an accelerated pace. After 12 yr of treatment, soil macroorganic matter in the top 5 cm of sediment was 475 g C m Ϫ2 lower in fertilized plots than in controls, which is equivalent to a constant loss rate of 40 g C m Ϫ2 yr Ϫ1. It is not known whether soil carbon in fertilized plots has reached a new equilibrium or continues to decline. The increase in soil respiration in the fertilized plots was far greater than the loss of sediment organic matter, which indicates that the increase in soil respiration was largely due to an increase in primary production. Sediment respiration in laboratory incubations also demonstrated positive effects of nutrients. Thus, the results indicate that increased nutrient loading of oligotrophic wetlands can lead to an increased rate of sediment carbon turnover and a net loss of carbon from sediments. Notes an oligotrophic salt marsh has significantly reduced the carbon standing stock and increased the turnover rate of soil carbon, which indicates that increased nutrient loadings could result in a net transfer of soil carbon to the atmosphere from organic-rich soils. This has important implications for global climate, to the extent that eutrophication may alter the carbon balance of wetland soils generally, and for maintenance of coastal wetlands threatened by rising sea level.

Estuarine Coastal and Shelf Science, 1997

Measurements of nitrogen, organic carbon and δ13C are presented forSpartina-dominated marsh sediments from a mineral marsh in SW Netherlands and from a peaty marsh in Massachusetts, U.S.A. δ13C of organic carbon in the peaty marsh sediments is similar to that ofSpartinamaterial, whereas that in mineral marshes is depleted by 9-12‰. It is argued that this depletion in δ13C of organic matter in marsh sediments is due to trapping of allochthonous organic matter which is depleted in13C. The isotopic composition and concentration of organic carbon are used in a simple mass balance to constrain the amount of plant material accumulating in marsh sediments, i.e. in terms of the so-called net ecosystem production. Net ecosystem production (∼2-100 g C m-2year-1) is a small fraction (1-5%) of plant production (∼2000 g C m-2year-1). This small amount of plant material being preserved is nevertheless sufficient to support marsh-accretion rates similar to the rate of sea-level rise.

Frontiers in Ecology and Evolution, 2020

Global Biogeochemical Cycles , 2022

Wetlands are considered to be among the most productive but vulnerable ecosystems (Kirwan & Megonigal, 2013; Su et al., 2021; Wen et al., 2019). Despite covering just 4%-6% of the total land area, wetlands hold approximately 450 Pg of the global soil carbon, representing 25%-30% of the terrestrial biosphere carbon pool (Kayranli et al., 2010). Coastal wetlands are a crucial sink in the global carbon cycle due to high sedimentation rate and burial of organic matter (Drake et al., 2015; Packalen et al., 2014; Zhang, Bai, et al., 2021), and it is estimated that coastal wetlands globally store at least 53.7 Tg C yr −1 (Wang et al., 2021). However, wetland habitats have been impacted around the world, and despite international initiative to protect these habitats (e.g., Ramsar Convention on Wetlands), wetland degradation and loss rate remain high in Asia (Davidson, 2014), potentially altering the land's carbon source-sink dynamics over different time and spatial scales (Mitsch et al., 2013).

Marine Ecology Progress Series, 2019

Blue carbon ecosystems, including salt marshes, play an important role in the global carbon cycle because of their high efficiency to store soil organic carbon (OC). Few studies focus on the origin of OC stored in salt-marsh soils, which comes from either allochthonous or autochthonous sources. The origin, however, has important implications for carbon crediting approaches because the alternative fate of allochthonous OC (AllOC), i.e. if it had not accumulated in the Blue C ecosystem, is unclear. Here, we assessed the origin of OC in two mainland salt-marsh sites of the European Wadden Sea, analyzing δ 13 C of topsoil (0-5 cm) samples, freshly deposited sediment (allochthonous source), and of above-and belowground biomass of vegetation (autochthonous sources). We tested for effects of geomorphological factors, including elevation and the distance to sediment sources, and of livestock grazing, as the most important land-use form, on the relative contributions of allochthonous versus autochthonous sources to the topsoil OC stock. A negative effect of distance to the creek on the relative contribution of AllOC was found at only one of the two salt marshes, probably 2 due to differences in micro-topography between the two salt marshes. Additionally, the relative contribution of AllOC increased with increasing distance to the marsh edge in areas without livestock-grazing, while it decreased in grazed areas. Our findings demonstrate that spatial factors such as surface elevation and distance to a sediment source, which have been found to determine the spatial patterns of sediment deposition, also are important factors determining the relative contribution of AllOC to topsoil OC stocks of salt marshes. Furthermore, we provide first evidence that livestock-grazing can reduce the relative contribution of AllOC to the soil OC stock. These findings thereby yield important implications for C crediting and land-use management.

Journal of Geophysical Research: Biogeosciences

High rates of carbon burial observed in wetland sediments have garnered attention as a potential "natural fix" to reduce the concentration of carbon dioxide (CO 2 ) in Earth's atmosphere. A carbon accumulation rate (CAR) can be determined through various methods that integrate a carbon stock over different time periods, ranging from decades to millennia. Our goal was to assess how CAR changed over the lifespan of a salt marsh. We applied a geochronology to a series of salt marsh cores using both 14 C and 210 Pb markers to calculate CARs that were integrated between 35 and 2,460 years before present. CAR was 39 g C·m −2 ·year −1 when integrated over millennia but was upward of 148 g C·m −2 ·year −1 for the past century. We present additional evidence to account for this variability by linking it to changes in relative sea level rise (RSLR), where higher rates of RSLR were associated with higher CARs. Thus, the CAR calculated for a wetland should integrate timescales that capture the influence of contemporary RSLR. Therefore, caution should be exercised not to utilize a CAR calculated over inappropriately short or long timescales as a current assessment or forecasting tool for the climate change mitigation potential of a wetland.

Current Forestry Reports

Purpose of Review We use the 'seascape' concept to explore how interactions between mangrove forests, tidal marshes and seagrass influence the storage of carbon in these ecosystems. Mangrove forests, with the other two 'blue carbon' habitats, are exceptionally powerful carbon sinks. Maintaining and enhancing these sinks is an emerging priority in climate change mitigation. However, managing any one ecosystem on its own risks is ignoring important contextual drivers of carbon storage emerging from its place in the seascape. We consider how interactions between these coastal habitats directly or indirectly affect the amounts of carbon they can store. Recent Findings The export of carbon from seagrasses may occur over hundreds or thousands of kilometres, much further than reported for mangroves or tidal marshes. Seagrasses may buffer mangroves from wave impacts, assisting forest regeneration. Trophic cascades supported by contiguous blue carbon habitat may limit excessive herbivory and bioturbation in them but evidence is limited. Summary Direct transfers of carbon between blue carbon habitats are common and are likely to enhance total carbon storage, but our understanding of their contribution to carbon stocks at the seascape level is elementary. There is evidence for indirect enhancement of carbon storage at the seascape by close association of habitats, mostly through the creation and maintenance of propitious conditions by one ecosystem for another. Protection from waves of mangroves by seagrass and protection from excess nutrients and sediment of seagrass by mangroves and tidal marsh are key mechanisms. There is little evidence or theory suggesting negative effects on carbon storage of one blue carbon habitat on another. Keywords Mangrove. Seagrass. Tidal marsh. Carbon. Seascape. Sequestration 'When we try to pick out anything by itself, we find it hitched to everything else in the Universe' (John Muir) My first summer in the Sierra,